Seismic prospecting



June 3, 1952 J. P. MINTON 2,599,064

smsuxc PROSPECTING Filed July 22, 1946 2 Sl-IEETSSl-IEET l INVENTOR. Jay/v P M/vra/v Mi GM A r rAwEr June 3, 1952 'J. P. MINTON ssxsmc PROSPECTING Filed Ju1 22 1946 2 Sl-lEETS-SHEET 2 T WF EPA 0053b N MF INVENTOR. Jaw/v P /V//A/ Ta/v 4 ORA/E Y Patented June 3, 1952 SEISMIC PROSPECTING John P. Minton, Dallas, Tex., assignor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application July 22, 1946, Serial No. 685,334

9 Claims. 1

Thi invention relates to seismic prospecting and particularly to systems in which signal channels respectively associated with the geophones of a spread are cross-connected so to effect mixing of the geophone signals to produce a composite record in which the reflections may be readily identified.

In seismic prospecting an explosive charge is detonated in a shothole in order to generate seismic waves for detection by geophones or other sensitive wave-responsive devices. As those skilled in the art know, considerable skill is required in interpretation of seismograms or records of the detected seismic waves to recognize reflections as distinguished from local disturbances which also affect the geophones. These disturbances arise because the explosion of the charge produces seismic waves not only within the frequency range of those which are reflected from subsurface strata, but also frequencies outside of that range. Moreover, there are present during the time of taking a seismogram many seismic waves which are not the result of a reflection from an interface but which represent the continued presence or local seismic energy within the frequency range of the reflections it is desired to record. By utilizating a plurality of geophones generally known as a spread, it is possible by comparin the several seismograms to recognize reflections. It has also been proposed to mix the signals from the several geophones so that the local and unwanted extraneous disturbances, including ground roll and wind strays, will be eliminated or greatly reduced in amplitude by reason of cancellation. Theoretically, the improvement obtainable by mixing should be proportional to the square root of the number of channels mixed. However, such an improvement has not previously been achieved in practical operation and the improvement realized does not seem to increase to an expected degree when the outputs of three and higher numbers of geophones v are mixed in one channel.

It is an object of the present invention to provide mixing arrangements which improve the seismograms by amounts more nearly proportional to the square root of the number of chanthose of seismic waves so to attain reinforcement of the signal frequencies of interest in the record, or they may favor the undesired signal frequencies which have mutually cancellin effects in the mixing because of phase differences, so to minimize in the composite record the frequencies corresponding with undesired disturbances: in either event, the cross-channel include a mixer tube for asymmetric passage between the signal channels of the frequencies passed by the filter.

While the mixture of two or more channels produces an improvement in a seismogram without the inclusion of said filters, further improvements are achieved by reason of the filters and therefore the invention is utilizable to advantage in the mixing of less than three channels as well as three and more channels.

The invention further resides in features of combination and arrangement hereinafter described and claimed.

For a more detailed explanation of the invention, and for further objects and advantages, reference is made to the accompanying drawings, in which:

Fig. 1 is a circuit diagram, in simplified form, of a geophone spread utilizing selected frequency mixing;

Figs. 1A and 1B illustrate alternative arrangements for controlling the magnitude of the mixing; and

Figs. 2-4 inclusive are explanatory figures referred to in discussion of the operation of the system shown in Fig. 1.

Referring to Fig. 1, each of the geophones IDA, IOB, IUC and MD, suitably spaced to form a spread along the area under survey, produces in response to reception of seismic wavesa signal output which is transmitted through a corresponding one of the signal channels A to D to the associated element of a recorder H, which produces a composite record which can be analyzed to determine the depth, slope and other information concerning subterranean strata. The signal output produced by the geophone IOA, for example, is impressed upon the input circuit of an amplifier having a suitable number of stages. For simplicity of explanation only a single stage is shown. 1

In the input circuit I2A of the amplifier is included a coupling transformer HA connected to the geophone. Preferably as shown, the control grid of the amplifying tube is connected to a potential dividin resistor ISA which may be manually adjustable to determine the percentage of the total available input voltage that is applied In the system as thus far described, the indi-" vidual geophones MA to 19D"res'pondnot only to the seismic Waves which it is desired to record but also to unwanted disturbancesnwhichare minimized, as appearing in the seismograph record, by the selective frequency cross-channel connections now describedn- The input terminals of a filter 8. are Zconneeted tott'hei signal channeliB fori imprfessionlthereoni or the signalsi iproduced by geophon'er. l 0B;" for ='examp1e, =the input circuit 0f the filter: may be' connected-in shunt to the'amture with the signals produced by geophone [0A of selected frequencies of the signals produced by geophone IOD.

Accordingly, by selective actuation of the switches I9, 23 and 28 there may be mixed in channel A selected frequencies of the signals produced by any one or more of the other geophones,

nnd'esired" components of "the' signal output pro duced by geophone IDA are insignificant compared to the desired frequency components as ,enhanced by the additive effect of the mixed channels.

rciz'whenthe cross-channel connection to a partic'ulariohe of theother channels is not desired, the -eorispoiiding*- switch I9, 23 or 28 is moved to its other irie'ffective position in engagement with contacts zl,-25 and 30 respectively to apply engagement with its contact'zagztheoutput signal :of thefilter is 'impressediupon' the input electrodes of La imdxing .tube 'whosez platez circuitis included in charmeltAw :The efrequeneies :of si'gnal phari'rifl B- which are assed by filter =18 are :injected into 1 signal: channel A: because the transmission ch'aracteristie; of the' cross channel is"asymmetriwdue to theLmi-x-ing tube; those same f-requenci'es are not *transmitted from channel f to channe1= B.

in: shortathe cross-channe1 1' including filter 8, as

well as Tithe: :other crossch'annelslater discussed, is frequency-selective'and asymmeti ic: l':he mi-x ing tube'may .beza separateitube, JOY-9S shownflriiay her-comprised in tube ZAwhich may forshoth plifying and mixing purposes ofia'typ'e {like lzz rTheifilter lfi'may be of the band-pass type having'a frequency response characteristic generally similar to that shown in Fig. 2. In such event,

,thelfilter l aselects-from the signal' 'output of the i geophone 18B a desiredaban'd: of fr'equencies', usually from about thirty cycles to seventy? cycles penzsecond corresponding: with those of seismic waves -nndersstudy and impresses-such frequency band upon the tube 12A for admixture withf all s1 enthe: signal-..freq:uencies produce'dby' the: geophone IDA-x5e: the refiected waves' froin deep strata arrive substantially. at} the sametime at the spaced 'geophone's of a spread, the reflected wave signal energyselected-by filter [8 from "char-1'- rich-Band introduced into channel'A -by themixing tube is? additive in channelto the reflected wave seismic signals produced by geophonei fl- =zs'flhe LSySten'iI shown also incluolesa filte'r 22 assohiatediiwith channel. C for selecting fr si-gnals i -:produced by geophone -'l 00 -the signal frequencies icorrespondingll with "those of the' s'eismiczvkaves: z'By, closure of switch:23 to -effect engagementzof its-:conta-ct 2A, the-output 'of the the - Channel D -may similarly be" "provided wit a predetermined fixed voltage, which as shown may be zero voltage, to the corresponding control electrode or grid 0f the associated mixing tube- :to avoid any significant chane of the operatingvoltaige of fotl-ier tubes usedfor'nmplifi :ca'tio'n or mmin'g' which are energiicd froxm'ime "samel ower-supplyi 1 h .VmGhanneL'B:lassociated with geophone IOB is similarly providedwith switches for selected admixture with the signals produced bygeophone -:l 0B 0f selected frequencies =of "the signals "p'roduced by any-one or moreof the other 'geeph'o'nes of the: spread.- For example; by movement pf switch contact 3-1out of engagement with the grounded contact 33=and into engagemenvwith contact 3 2-; the output'of athe filter' "2 2 is P applied toapair of-inputelectrodes of the mixemtu-be 12B, thus :to mix with -the"signa-l =in' ch'annel 'B the selected frequencies of-theisignal'preduc'ed by geophonerlOG; -;Similarly-, by actuation-of 'sw-itch contact -34--from engagement with the fixed 'cofitact,i z(i and: into engagement with contact35', the filter '2 1 is connected: to the -input'-'e=lectrodes"*o'f one section of mixertube -2 6B foradmixtui'e'with the signals in: channel' -B of the selected Eirequencies of the signals produced by geophohe -i.0D.- In like manner, the.-switch--comprisingeon tacts 31 %3 8 1 and 39- may be actuated for' mixture with the signals in channel -B"of the'selected frequencies ofethe signals produced by 'geoplfon [0A- or,* ina :geophone' spread having more than four -geophones, of the 'selected frequendieshf any one of-the additional channel's =notshowna -I-n #generally"like m'anner, to the sigiial's' 'produced-bygeophone C there may'-be"added the selectedfrequencies of thesigzia-ls in anyone or more I of the other channeIsa-thuS *efiectivefy to minimize in the resulting composite signal output the undesired frequencies' produced by the ge e phone I 0G. For "example; selected frequencie's l-n thesignal output of geophone I 0D may be'intro' duced'i'nto-channel Cby closur ofswitch fll "d reflect-engagement of'contact'fl'li The' 'frequeh" es selected'byz-filte'r 21 are impresseduporia*p'aiIWSf input electrodes of the'second sectionbftu 2C", or of a physically separate-mixer tube',' wh'o'se fit put circuitis included in* chaniiel*O. "In e manner,- the switches comprising contacts 43;1'1"? and 46/41; 48'may be selectively actuated fifr connection to the se'ctionsof the mixer 'tub'flw the filters associated with other 'channe'ls' 'of spread, only 'lead lines ofwhich are 'showz i.-'

mixing tubes and cross-channel selector" -itlfhes- 49'-'-5 1-; 52:54 and-55*5l' providingfo adm ture with the signals produce'd by the geopfi I0 of1 heselected-frequ ciese which the 5, any one or more of the other signal channels of the spread.

By the arrangement described, the signal out put of any one or more of the channels may to selected extent include the seismic signals of the geophone of that channel and anyone or more of the other channels with additive effect only of the signal frequencies corresponding with seismic waves of a desired frequency band and with no more signal component due to extraneous disturbances, such as ground roll and wind strays, than would be present without mixing. So far as the resulting record is concerned, the effect is that of enhanced prominence of'the curve characteristics relied upon in determination of the depth and slope of subsurface strata.

With the construction of variable resistors l3A-l3D shown in'Fig. 1, the positions of their adjustable contacts control both the amount of mixing and also the magnitudes of unfiltered signals applied to the amplifiers of the respective channels. To afford additional flexibility by independent control of the mixing and the un-" filtered signal input, the resistors ISA-ND may I be replaced by either of the arrangementsshown in Figs. 1A and 1B. In the former, the input lead to the corresponding filter I8, 22 or 21 is connected to the movable contact of oltage-dividing resistor 13 whereas the input lead to the am plifler tube I2B, I2C or |2D is connected to the movable contact of a second voltage-dividing resiswr SIS in shunt to resistor I3. The same independence of control, which is of advantage under circumstances such that a reflection would not otherwise carry through on all of. the traces, may be obtained by providing each of the resistors I3AI3D with two adjustable contacts, as shown in Fig. 13, connected respectively to the associated filter and to the associated amplifier tube. 4

It is also possible to produce records in which the unwanted signal disturbances are minimized by using band-elimination filters having gen-.

cancellation. In any event, the overall effect sofar as the ultimate record is concerned, is generally the same as that attained by filters of the band-pass type, namely, emphasis in the record of the seismic wave signals and minimization of signals due to undesired disturbances.

The cross-channel filters I8, 22 and 2! may each be of type disclosed in my co-pendingappli- 6 waves whose predominant frequency changes during the recordinginterval.

While-the mixing of four channels has been shown in'detail, it is to be understood that the output signals from any desired number of geophones may be mixed together in accordance with the invention and that in each case there will be further improvement of the seismogram in the elimination of the local disturbances and in the emphasis of the reflected waves which arrive substantially simultaneously at the several geophones in avoidance of the cancellation effect which is enhanced by reason of the present invention.

While a preferred form of the invention has been described and illustrated, it is to be understood modifications may be made within the scope of the appended claims.

What is claimed is:

1.' A seismic prospecting system comprising at least two geophones for producing signals in the seismic frequency range of about thirty to seventy cycles per second in response to reflections of a shot impulse from subsurface strata and also responsive to local seismic disturbances resulting from said shot impulse to produce undesired signalsfa corresponding number of recording means, signal, channels each between one of said geophones and one of said recording means, and at least one frequency-selective asymmetric crosschannel connection including a filter and a mixing tube providing for asymmetric passage of selected frequencies from one to another of said channels, thereby in advance of its application cation, Serial No. 679,991 whose frequency response as shown by curves Tl, T2 and T3 of Fig. 4, automatically varies as a function of time in the interval following a shot so that the frequencies selected from the signals of individual channels for admixture with the geophone signals of one or more of the other channels shift with time continuously to favor the reflections from deeper shot impulse from subsurface strata and also re-- sponsive to.loca1.seismic disturbances resulting from said shot'impulse to produce undesired signals, a corresponding number of recording means, signal' cha'nnels each between one of said geophones and one of said recording means, and at least one frequency-selective asymmetric cross-..

channel connection including a band-passfilter and a mixing tube providing for. asymmetric passageof desired seismic signal frequencies from a first to a second of said channels for cumulative effect upon. the recorder associated with said second channel of the same desired seismic signal frequencies impressed upon both of said channels by the geophones respectively associated therewith. I

3. A seismic prospecting system comprising at least two geophones for producing signals in the seismic frequency range of about thirty to seventy cycles per second in response to reflections of a shot impulse fromsubsurface strata and also' responsiveit'o' local seismic disturbances resulting from said shot impulse to produce undesired signals, a corresponding number of recording means, signal channels each between one of said geophones and one of said recording means, and at least one frequency-selective asymmetric crosscha-nnel connection including a band-elimination malls s firs $9 a. ee? d. I.

flier-e229 s siev Providing-40f emcee" e seeeiiecesl ie preemies s e alssis mc-ic psssage' of 'unde l the seismic'frequency range of aboutt ys sa om let In .5 P21} cl eeeiione other-cf said channel selec risewii i he means.

18. seismic ntOSWcti-ng system comprising a lura ity of fi phonespfor producing signals in th seismie frequency ran e or about thirty .to ,seyenty. cycles per-..seconel.in itesponse to reflections of a shot impulse from subsurface strata resulting from said shot impulse to produce uncle sired gnals a eoi fiesponding numberof record ingn-ieans, signal. channels each. between one o! geophones and one of said recording means, and mfians for impressingvthe same form ofdecles oi the g 5 siredsigpalenergy'irem each of one or more one; M v 012531;]; signal ehannels upon at: least one other- ;iipduce d b l r ef-said signalchannels comprising a plurality-of asymmetric -frequency-selective cross-channels each including a. band-pass filtenselectivelyalinjuring passage. of the desired signal frequencies and a mixing tube, and cross-channel selector v,sizcitehing means to enhance the frequencies corr pondin withreflections from subsurface strata.

:9 A seismic prospecting system comprising I a pluralityv of geophones for producing signals in the seismic frequency range of about thirty to seventy cycles persecond in response to reflections I of- .a shot. impulse fromsubsurface strata and also responsive to local-seismic disturbances resultingfromsaicl shot impulse to produce un-- desired signals a-cori esponeiing'number of recording means, signal channels each between one ofsaid geophones and one of said recording means, and meansfor-impressing undesired sig-- channeis uponat least one-other of said signal channel'scomprising a plur ality of asymmetric f equency selectiue cross-kchannels each including a banci elimination filter selectivelyallowing; passage of the undesired signal-frequencies- -and a i mixes-tube, and cross-ohanneh selector switching means,- tov -1;edu oe-;the frequencies corresponding with localseismic-disturbances.

' flGI-IN'P; 'MI NTON.

REFERENCES CITED he-fo1-lowing references are of record in the file of this patent;

UNIIED AT BATE S sea freouency-selective asymmeti ic cross-c nelconnect'ions, each including a filtec a a mixing tube, jointly providing for asymmet i c passage to each ct said channels of the same selected seismic frequencies from at least'one Name Date othecofi-saidfchannels, to enhance. insaici i-ecord- Wcis-,;Jr= Apr. V21, 1936.- ingmeansthe frequencies corresponding with Phelps i Feb. 10, 1942' reflections i-zon subsurracestratau hazel Dixon Sept; 14, 1943 v {i seismic prospecting system comprising; in 70 asm-,-s2e Hazlett g... Mar. 14', 1-950:

Y ngl r and a mixmg tub and cross:-

and @159 IfiSDonsive to local seismic disturbances nal-enevg-y from-each of one or more of said signal- 

